1. Field of the Invention
This invention relates to a fuel cell, and more particularly to an fuel cell in which gas leakage is reduced, the deformation of the electrode is prevented, and the assembly cost is reduced.
2. Description of the Related Art
Air contamination caused by exhaust gases from automobiles is a critical problem. Electric vehicles have been viewed as a solution to this problem since they do not produce air contaminants at an exhaust pipe. Yet, electric vehicles have not seen consumer acceptance, in part because of their limited range and the necessity of frequent recharging of their batteries.
In recent years, automobiles equipped with fuel cells generating electricity by reverse reaction of electrolysis using hydrogen and oxygen have been developed as environmentally acceptable vehicles which discharge only water into the atmosphere. A solid polymer electrolyte type fuel cell operating at low temperature is the most promising type for use on automobiles.
The solid polymer electrolyte type fuel cell is generally structured as a stack of many single cells. A stack of such cells comprises an electrode unit including a solid polymer electrolyte membrane sandwiched by two electrodes (a fuel electrode and an oxidant electrode), and separators having gas flowing conduits for the fuel gas or the oxidizer gas sandwiching the electrode unit.
The following reaction occurs by the contact of the hydrogen in the fuel gas with the catalyst at the fuel electrode:
2H2xe2x86x924H++4exe2x88x92.
H+is transferred through the solid polymer electrolyte membrane to reach the oxidant electrode catalyst and to generate water by combining with the oxygen in the oxidizer gas. That is:
4H++4exe2x88x92+O2xe2x86x922H2O.
A large volume of gas is required to be used in the catalysts to perform the above described reaction effectively. Leakage of the fuel gas and the oxidizer gas at any place except the electrode contacting portion must be prevented. To this end, Japanese Patent Laid open Publication H 09-17437 (published on Jan. 17, 1997) discloses a gasket disposed between separators and a solid polymer electrolyte membrane adhered to the electrodes. Similarly, WO 92/22096 discloses a gasket adhered to electrodes and a solid polymer electrolyte membrane in one unit overlapped with the electrodes and the solid polymer electrolyte membrane.
However, the conventional fuel cell structures have certain disadvantages. Since a large number of parts is required in Japanese Patent Laid-open Publication H 09-17437, the gasket is structured as one unit with the separator. Generally, the rubber gasket is printed on the separator by injection molding or compression molding to make the gasket and the separator into one unit. In this case, some means is required to prevent the flow of the gasket forming material such as rubber into the conduit introducing and discharging the gas from a manifold to a cell. The technical means employed for this is to mold the conduit under a nest, and to remove the nest after molding. However, it is difficult to completely stop the flow of the gasket material into the conduit, and so an additional process to remove the nest is required. This is a fatal drawback for mass production.
In the structure of WO 92/22096, where the gasket is overlapplingly adhered into one unit with the electrode and the solid polymer electrolyte membrane, the gas leakage can be reduced by covering the solid polymer electrolyte membrane with the electrode and the gasket when the gasket is assembled in an overlapped state with the electrode and the solid polymer electrolyte membrane. However, a predetermined form cannot be obtained without a relatively large load being applied on the electrode when it is adhered with adhesive agent in the actual producing process, which may lead to the deformation of the electrode and deteriorate the power generation capability.
Since the gasket is here adhered to a part of the solid polymer electrolyte membrane at a distance from the electrode contacting portion, there may be gas leakage from the solid polymer electrolyte membrane between the electrode contacting portion and the gasket.
Accordingly, it is an object of this invention to provide a sealing structure of a fuel cell electrode with less gas leakage, with no deformation of the electrode, and which has a low production cost.
According to a feature of the invention, the above and other objects are achieved by a fuel cell having an electrode unit comprising an oxidant electrode, a fuel electrode, and a solid polymer electrolyte membrane sandwiched by the oxidant and fuel electrodes, the solid polymer electrolyte membrane having a larger area than the oxidant and fluid electrodes, whereby the solid polymer electrolyte membrane has an electrode contacting portion and an outer peripheral portion projecting from the electrode contacting portion; and a gasket covering the outer peripheral portion of the solid polymer electrolyte membrane.
Since the gasket completely covers the entire periphery of the solid polymer electrolyte membrane, gas does not leak from the solid polymer electrolyte membrane and the gas leakage can be reduced. Also, since the gasket does not overlap the electrodes, deformation of the electrodes does not occur.
In addition, since the solid polymer electrolyte membrane, the electrodes and the gasket are in one unit, it is possible to reduce the number of parts. In consequence, the assembly of the fuel cell is facilitated, the assembling time can be reduced, and the production cost for the fuel cell can be reduced.
According to another feature of the invention, a reinforcement portion is disposed in the periphery of the gasket. Since the shape of the gasket is maintained by the reinforcement portion, the handling of the gasket during assembling is facilitated, the assembling time can be reduced and the cost for producing the fuel cell can be reduced. When the solid polymer electrolyte membrane is stacked with the electrodes as a unit, the deformation and destruction of the sealing portion due to the deformation of the gasket can be prevented.
According to another feature of the invention, respective supply holes and discharge holes for the fuel gas, the oxidizer gas and the cooling water are provided in the gasket. The fuel cell includes the gasket between the separators, a supply manifold and a discharge manifold formed from the supply holes and the discharge holes provided on the gasket, and fluldically communicating with the supply holes and the discharge holes for the fluid (the fuel gas, the oxidizer gas, and the cooling water) provided on the separator, which enables sealing of the fluid.
According to another feature of the invention, a bead shaped projecting portion encloses at least one of the electrode contacting portion, the supply holes and the discharge holes in the gasket. The fluid can thereby be tightly sealed by the contact of the bead shaped projecting portion to the separator.
According to another feature of the invention, a reinforcement portion is disposed in the gasket and is composed of a continuous or intermittent wire or a thin rectangular plate enclosing the solid polymer electrolyte membrane. Since the entire gasket can be reinforced with a simple structure, an excellent reinforcement capability and lower cost can be assured.
According to another feature of the invention, the gasket is elastic, and the gasket and the projecting portion are formed in one unit. Since the gasket is elastic, it can fully contact the separator without allowing any gap therebetween, and the fluid can be completely sealed. Since the gasket and the projecting portion can be produced in one process, the producing cost can be reduced.
According to another feature of the invention, one of the bead shaped projecting portions disposed to enclose the electrode contacting portion is provided inside the peripheral portion of the solid polymer electrolyte membrane. Since the bead shaped projecting portion compresses the solid polymer electrolyte membrane, the solid polymer electrolyte membrane is tightly sealed to the gasket.
According to another feature of the invention, the gasket formed as one unit. Since the gasket is structured as one unit, the fluid can be tightly sealed. Also, the production cost can be reduced since the gasket is produced in a single process.
According to another feature of the invention, a portion covering the peripheral portion of the solid polymer electrolyte membrane on the gasket is divided into two portions at the surface of the solid polymer electrolyte membrane. The division of the portion covering the solid polymer electrolyte membrane enables it to be bonded with an adhesive agent.
According to another feature of the invention, the entire gasket is divided into two substantially identical parts at the surface of solid polymer electrolyte membrane. The division of the portion covering the solid polymer electrolyte membrane enables it to be bonded with an adhesive agent.
According to another feature of the invention, at least one of lid portions of a gas inlet and a gas outlet supplying and discharging the fuel gas and the oxidizer gas respectively from the supply holes and the discharge holes of the fuel gas and the oxidizer gas to the electrodes is connected or attached to the gasket fluid-tightly. Connecting or adhering lid portions and the gasket simultaneous with the assembling of the fuel cell enables shortening the assembling time and reducing the production cost.